International Journal of Fatigue 37 (2012) 1–7

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International Journal of Fatigue

journal homepage: www.elsevier .com/locate / i j fa t igue

The influence of hydrogenated amorphous carbon coatings (a-C:H) on thefatigue life of coated steel specimens

Jens Schaufler, Karsten Durst ⇑, Thomas Haas 1, Roland Nolte, Heinz Werner Höppel, Mathias GökenUniversity Erlangen-Nürnberg, Department of Materials Science and Engineering I, Martensstr. 5, 91058 Erlangen, Germany

a r t i c l e i n f o

Article history:Received 3 March 2011Received in revised form 2 August 2011Accepted 18 September 2011Available online 29 September 2011

Keywords:a-C:HEnhanced fatigue lifeCoatingReduced surface roughening

0142-1123/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.ijfatigue.2011.09.008

⇑ Corresponding author. Tel.: +49 9131 85 27505.E-mail address: Karsten.durst@ww.uni-erlangen.d

1 Present address: KIT, Karlsruhe, Germany.

a b s t r a c t

The influence of a hydrogenated amorphous carbon coating on the fatigue life of an austenitic steel wasevaluated in this work. The fatigue damage mechanisms have been compared for both coated anduncoated specimens. A strong increase in fatigue life of more than 300% for low total strain amplitudeswas found for the coated specimens. The influence of the coating on the fatigue life increases withdecreasing total strain amplitude.

The damage mechanisms were analyzed using atomic force microscopy and focus ion beam cross sec-tioning. The coated specimens show a significantly reduced surface roughening. Slip bands in the steelsample could not penetrate the coating and thus slip step formation on the surface was avoided, leadingto the enhanced fatigue life. The compressive residual stress in the coating is discussed being the mainreason for the enhanced fatigue life of the specimens.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction or a multilayer setup [18], the interfacial strength between the

It is well established, that surface modifications as shot peening,case hardening or the deposition of thin hard coatings on ductilesubstrates can lead to a significant improvement of the fatigue lifeof components under cyclic loading conditions [1,2]. For workhardened surfaces, compressive residual stresses are discussed toincrease the material stability against fatigue damage [3–6].

Compressive residual stresses in the surface either delay or hin-der crack initiation and will also retard crack propagation. For thinhard coatings, depending on the coating as well as the interface be-tween coating and substrate, either a strong reduction or enhance-ment of fatigue life is found in literature [7–13].

The following aspects are discussed to lead to a beneficial effectof the hard coating on the fatigue life of the coated specimen:

� high interfacial strength between the coating and the substrate� a relative large strain to failure for the coating� high residual compressive stresses in the coating

These properties apply essentially to hydrogenated amorphouscarbon (a-C:H) coatings. a-C:H coatings usually contain high com-pressive stresses in a range of 1–5 GPa [14,15]. Depending on thecoating structure the maximum elastic tensile strain is larger than1% until fracture occurs [16]. By using metallic adhesion layers [17]

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e (K. Durst).

substrate and the a-C:H coating can be improved.Based on the available literature a general positive effect of an

a-C:H coating on the fatigue resistance of steel cannot be stated.Golden et al. [19] and Sundaram [20] could not detect any signifi-cant effects of a-C:H coating on the fatigue life. Puchi-Cabrera et al.[21] and Morita et al. [22] however found a strong improvement infatigue life for a-C:H coated specimens. Most of the work focusedon the S–N plots with only limited microstructural investigationon the damage mechanisms. However, no detailed investigationof the interaction of fatigue damage with the substrate-coatinginterface was carried out.

In this work total strain controlled fatigue tests were performedon a-C:H coated and uncoated austenitic stainless steel up to amaximum of 2.3 � 105 cycles to failure. The deformation and dam-age evolution of the two samples was investigated in terms of sur-face roughening, slip bands, crack initiation and crack propagationusing Atomic Force Microscopy (AFM) as well as Focused Ion Beam(FIB) cross sectional analysis. The damage evolution in the coatedand uncoated specimens at similar number of cycles to failure iscompared in detail. By doing this, an insight in the interaction be-tween fatigue shear bands and the industrial a-C:H coating system,consisting out of an Cr-based adhesion layer and a functional a-C:Hlayer is given. The fatigue life curves are finally discussed with re-spect to the residual stress in the coating.

2. Experimental setup

This study was performed with coated and uncoated samples ofthe 1.4541 (DIN) austenitic stainless steel. The steel contains in

2 J. Schaufler et al. / International Journal of Fatigue 37 (2012) 1–7

addition to C, Cr and Ni, Ti with a maximum amount of 0.8 wt.% tostabilize the microstructure against intergranular corrosion inaqueous environments. The specimens were machined to a typicalround dog-bone shape (see Fig. 1).

The required smooth surface for fatigue testing was achieved bygrinding, polishing and a final electrolytical polishing step with theAC2 Struers electrolyte at a voltage of 60 V. The specimens exhib-ited a smooth surface, with an average RMS-roughness of 18 nm asdetermined by atomic-force microscopy (AFM) 10 � 10 lm scans.Close to Ti-rich precipitates, most probably a Ti-carbide, preferen-tial etching occurred, leading to some surface defects (see Fig. 2).The deposition of the 2 lm a-C:H coating – including a Cr-basedadhesion layer between the steel and the a-C:H – was performedin a Balzers BAI830 PVD/PECVD system. Details about the deposi-tion process, the mechanical properties and the microstructuralsetup of the coating system can be found elsewhere [16]. The coat-ing has a compressive residual stress in a range of�2 GPa, as deter-mined by beam curvature tests. The mechanical properties weredetermined with nanoindentation. The measurements reveal ahardness of 23 GPa and a Young’s modulus of 209 GPa for the a-C:H coating. Rockwell C adhesion tests (VDI standard 3198) yieldan adhesion class of 1–2, indicating an excellent adhesion strengthbetween the substrate and the coating.

After the coating process, a slightly enhanced surface rough-ness is found, but the coating nicely covers the surface defects(see Table 1 and Fig. 2).

The symmetric push–pull fatigue tests (R = �1) were carried outin ambient air at room temperature on a servohydraulic MTS Sys-tem 810 with a maximum load capacity of 100 kN. All tests wereperformed under total strain control at a frequency of 4 Hz. All fa-tigue tests were performed until failure occurs or when 2.3 � 105

cycles were achieved. For the latter case the specimens weredenominated as ‘‘specimen without rupture’’. In addition a pre-straining of the coated specimens was done to a tensile strain levelof 1.5%. These specimens were fatigued to fracture to investigatethe influence of pre-cracking of the coating on the fatigue lifeand on the damage mechanisms.

Fig. 1. Specimen geometry (Dimensions in mm).

Fig. 2. FIB cross section of a coated surface defect. The EDX elemental map depictedas RGB image (red: Fe, green: Ti, blue: Cr) reveal a high amount of Ti in theprecipitate beneath the surface defect. (For interpretation of the references to colorin this figure legend, the reader is referred to the web version of this article.)

After fatigue testing, the surface roughening of the differentspecimens were investigated with an Atomic Force Microscope(AFM) 3100 from Digital Instruments. Focused Ion Beam cuts andScanning Electron Microscopy (SEM) were performed in a ZeissCrossbeam 1540 EsB.

3. Results

3.1. Fatigue data and microstructural evolution

In Fig. 3 the cyclic deformation curves for two uncoated andcoated specimens fatigued at low and high strain amplitudes(3.25 � 10�3/3.35 � 10�3 – low amplitude and 4.4 � 10�3 – highamplitude) is exemplarily shown. Both coated and uncoated spec-imens exhibit a cyclic strain hardening behavior, where the stressamplitude is not affected by the coating. The stress amplitude ismainly governed by the bulk sample, without a large influence ofthe 2 lm thick a-C:H coating. The number of cycles to failure ishowever influenced by the coating: For the high total strain ampli-tude a moderate increase in lifetime of 20% is found, whereas atsmall amplitudes the lifetime of the coated specimen is increasedby more than 300%.

Fig. 4 shows the fatigue life diagram, where the total strainamplitude is plotted as a function of the cycles to failure Nf forthe coated and uncoated specimens. For higher total strain ampli-tudes, only a small enhancement in the fatigue life time is foundfor the coated specimens. However, at small total strain ampli-tudes below 4 � 10�3 the a-C:H coated specimens show a signifi-cant enhancement of the fatigue life up to 400%. For thespecimens without rupture, marked in the plot, an increase ofthe total strain amplitude of �1 � 10�3 is obtained. The resultsindicate a strong improvement of the fatigue life for small totalstrain amplitudes with mainly elastic deformations, whereas atlarge plastic strains, the damage behavior is not strongly affectedby the a-C:H coating.

The AFM-surface analysis for uncoated and coated specimens attwo strain levels is shown in Fig. 5 (see also Table 1 and markers inFig. 4). In the AFM-micrographs clear intersections of slip bandswith the surface are found. The uncoated specimens UC1 andUC2 show a pronounced surface roughening after fatigue loading.The specimen UC1, fatigued at a total strain amplitude of4.50 � 10�3 shows a substantial number of large intrusions andextrusions, most of them oriented in 45� to the loading axis. Atthe smaller strain amplitude of 2.47 � 10�3 (UC2) a similar behav-ior is found. The observed large surface steps with the fragmentedheight profile indicate a strong interaction of the bulk fatigue slipbands with the specimen surface. In contrast, the coated specimensC1 and C2 show a significantly reduced surface roughening, withonly small intrusions and extrusions. Similar to the uncoated sam-ples, these extrusions are oriented in 45� to the main stress axis.The quantitative results of the AFM analysis are also given inTable 1, where a clear reduction in surface roughening is foundfor the coated specimens. Large extrusions with a height of morethan 50 nm are not found for the coated specimens. The detectedaverage extrusion height of around 30 nm remains slightly abovethe level of the surface roughness of the initial state.

The FIB cross sections reveal a strong localized deformationalong shear bands for the uncoated condition (see Fig. 6a). Withinthe shear bands a very small substructure is found. The shearbands intersect the surface at an angle of 45�. In contrast, the shearbands in the coated specimen C2 (see Fig. 6b) do not penetrate thesurface, but discontinue in the sputtered Cr structure of the adhe-sion layer. Only a small roughening (see markers: small buckles) atthe interface between the ramplayer and the a-C:H coating isfound. This is in good agreement with AFM surface observations,

Table 1Quantitative results of the surface roughening for the uncoated and coated specimens: initial state, low and high strain amplitude determined by AFM.

Uncoated (UC) Coated (C)

Total strainamplitude

RMS in(nm)

Maximum extrusionheight in (nm)

Average extrusionheight in (nm)

Total strainamplitude

RMS in(nm)

Maximumextrusion height in (nm)

Average extrusionheight in (nm)

Initial state – 18 – – – 28 – –High strain

amplitude (1)4.50 � 10�3 78 ± 15 342 150 ± 34 4.37 � 10�3 45 ± 10 47 33 ± 11

Low strainamplitude (2)

2.47 � 10�3 84 ± 9 307 149 ± 66 3.35 � 10�3 42 ± 6 50 29 ± 12

Fig. 3. Cyclic deformation curve for two different total strain amplitudes (high –4.4 � 10�3 and low – 3.25 � 10�3/3.35 � 10�3). The curves show similar stressamplitudes for similar total strain amplitudes. The coated specimen shows a strongincrease in lifetime for the low total strain amplitude. At the higher total strainamplitude of 4.4 � 10�3only a small increase in lifetime is found for the coatedsample. (For interpretation of the references to color in this figure legend, the readeris referred to the web version of this article.)

Fig. 4. Total strain fatigue life diagram for the coated and uncoated specimens. Thecoated specimens show a strong increase in lifetime for total strain amplitudessmaller than 4 � 10�3. For the pre-strained specimen, fatigued at a total strainamplitude of 3.5 � 10�3 a strong decrease in the fatigue life of nearly one order ofmagnitude is found. The specimens investigated by AFM and FIB are marked in theplot with black circles. Runouts are indicated by horizontal arrows.

Fig. 5. AFM surface topography of the fatigued specimens. The micrographs (a) and(c) reveal a strong surface roughening of the uncoated specimens UC1 and UC2. Theweak surface extrusions of the coated specimens C1 and C2 are shown in themicrographs (5b) and (5d). The exemplarily chosen AFM section analyzes show thedistinct roughness profiles. The loading axis lies horizontal for all micrographs.

J. Schaufler et al. / International Journal of Fatigue 37 (2012) 1–7 3

which also showed only a small roughening of the a-C:H coating(see Table 1). These surface steps in the a-C:H coating are clearlyrelated to the underlying slip bands and small buckles at the inter-face. Furthermore, by a so called FIB-freecut, it could be shown,that the a-C:H coating has deformed plastically – no elastic

relaxation of the surface can be detected (see markers in Fig. 7).Hence, the slight surface extrusions in the a-C:H are permanentdeformations. This indicates a microplastic shear band formationin the a-C:H coating, triggered by the fatigue shear bands in thesubstrate. In this context, it has to be mentioned, that shear bandsare also found in literature for other amorphous materials [23], butare not discussed in terms of an interaction between fatigue dam-age and amorphous carbon coatings.

3.2. Damage formation and fracture analysis

The surfaces of the fractured specimens show clear evidence offatigue cracking. Typical lentoid fatigue crack propagation is found,exemplarily shown in Fig. 8. The cracks form for the uncoated aswell as for the coated samples at surface defects. This is exemplari-ly shown for the coated specimen in Fig. 8b. The main crack, caus-ing failure of the specimen, started at a defect close to theunderlying Ti-precipitate. Local stress intensities are known tocause cracking as already reported in detail by Borbély et al. [24].Here the pore in combination with the brittle carbide leads to astrong stress intensity, causing failure of the coating.

Fig. 6. FIB cross sections of the tested specimens UC2 and C2: (a) The uncoated specimen UC2 shows pronounced shear banding along 45� to the loading axis. Within theshear bands cell structures have formed. (b) The cross section of the coated specimen C2 shows also shear band formation but reveals only small buckles at the interface ramplayer/a-C:H coating.

Fig. 7. FIB freecut of an a-C:H coating after fatigue loading. No elastic relaxation ofthe surface roughening is found. The markers show the permanent surfaceroughening, which is also found in the AFM sectional analysis (Fig. 5b and d).

Fig. 8. Top view of the fracture surface: (a) Typical lense shaped fatigue crack

Fig. 9. (a) The uncoated specimens show fatigue cracks which started at the in- or extru

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Besides cracking from pores, other damage and cracking modesare found for the uncoated samples. As shown before, the severeslip band formation in the bulk material leads to strong surfaceroughening of the uncoated specimens (see Figs. 5 and 6), followedby the initiation of typical fatigue cracks at the in- and extrusionson the surface (see Fig. 9a). In contrast, for the coated samplescrack initiation on the weakly pronounced extrusions on the a-C:H surface is absent. Only cracking at the pores/precipitates, asshown in Fig. 9b, can be found. Below the surface, cracking mightalready start as can be seen in Fig. 10 in the cross section of ana-C:H coated steel after 1.35 � 105 cycles at 3.3 � 10�3 total strainamplitude. Crack coalescence was observed close to shear bands inthe steel substrate and the Cr-layer. The cracks open within thesteel and Cr, but stop at the interface to the ramp layer.

After pre-straining the specimens to 1.5% tensile strain, brittlecracks are formed in the coating perpendicular to the main loadingaxis. The sharp cracks run straight through the coating and stop atthe steel substrate, causing there a notch effect [16]. The fatigue

propagation is found. (b) Crack nucleation occurred close to a precipitate.

sions, respectively. (b) For the coated specimens only cracks at pores were detected.

Fig. 10. Cross section of the coated specimen C2. Near the interface steel/Cr thecrack is widely opened. The crack closure in the region of the ramp layer indicates acompressive stress level in the upper region of the coating system, namely ramplayer and a-C:H.

Fig. 11. The pre-cracks in the a-C:H coating induced by the tensile deformationtrigger the fatigue damage of the coated specimens. The main crack follows the pre-defined crack path of the cracks in the a-C:H coating.

Fig. 12. Total strain amplitude as a function of cycles to failure. The total strainamplitude of the uncoated specimens is shifted by a constant strain amplitude of+7.6 � 10�4. The black curve shows, according to Eq. (1), the difference of the totalstrain amplitude Detot/2 for the two fits of the a-C:H coated and uncoatedspecimens (from Fig. 4), starting at Nf > 6000.

J. Schaufler et al. / International Journal of Fatigue 37 (2012) 1–7 5

tests of the pre-strained specimens reveal a strong decrease in fa-tigue life (see Fig. 4). Fig. 11 shows the fatigue crack of the 1.5%pre-strained (total strain amplitude 3.3 � 10�3) specimen. Thecracks in the a-C:H coating induced by the tensile deformation trig-ger the fatigue damage of the coated specimens. The main fatiguecrack follows the pre-defined crack path of the cracks introducedby the tensile pre-deformation of the a-C:H coating. Plastic pre-straining can also lead to a reduction in the compressive residualstress of the coating, which can also have an influence on the fati-gue life of the coated specimen [25].

4. Discussion

In order to understand the observed results better, the localinteraction between fatigue shear bands and the a-C:H coating,the local stress distribution at the surface needs to be considered.The presence of surface defects (see Fig. 2) leads to a reductionin fatigue life for both coated and uncoated specimens. However,still a strongly enhanced fatigue life and fatigue damage resistanceis found for the coated specimens. Furthermore, the a-C:H coatingprotects the underlying material against environmental effects likeoxidation, which is not the case for uncoated specimens [26,27].

A thin coating cannot influence the stress amplitude of a largebulk sample during cyclic loading. The stress amplitudes are thus

the same for both coated and uncoated specimens (Fig. 3). Fatiguedamage, however is initiated from the surface of the specimensand coated samples gain a large increase in lifetime. As has beenseen in the AFM and FIB analysis, the a-C:H coating strongly hin-ders the propagation of the shear bands, which is thought to bethe main mechanism for the observed enhanced lifetime.

One possible explanation for this mechanism is the largeamount of compressive residual stresses acting in the coating. Asimple approximation with Hooke’s law (compressive residualstress of �2 GPa, Young’s Modulus of 209 GP) reveals a compres-sive residual strain of eres � �0.95% in the coating. Compressiveresidual stresses are known to hinder crack initiation [5] and it istherefore expected, that the coated specimen requires a largerstrain amplitude for fatigue failure. The benefit in fatigue lifetimeis estimated to be proportional to the strain level in the coating,were tensile strains decrease and compressive strains increasethe fatigue life time (Eq. (1)).

To verify this model, the difference of the total strain amplitudefor the coated Detotal

2 coated

� �and uncoated Detotal

2 uncoated

� �speci-

mens is plotted in Fig. 12. There it is found, that the difference be-tween the two data sets is relatively constant at Nf > 30,000. At thefatigue strain limit, a difference of keres = 0.076% strain is found.

Detotal

2 coated¼ D

etotal

2 uncoated� keres ð1Þ

The strain level of 0.076% is now used for shifting the totalstrain amplitude of the uncoated sample Detotal

2 uncoated (Eq. (1)).As expected, the two curves now overlap at Nf > 30,000, while atsmaller Nf, the total strain amplitude of the uncoated sampleDetotal

2 uncoated is overestimated. The fatigue life at large strainamplitudes is dominated by the surface defects in the specimens,so that the residual stress in the coating has only a small beneficialeffect for the lifetime.

Comparing the found strain shift with the residual strain eres inthe coating, a factor of k = 0.08 is found. The fatigue lifetime of thecoated specimen does thus not fully benefit from the residualstrain in the coating. This can be understood by considering thecomplex residual stress field with tensile components underneaththe coating. Crack growth is accelerated in the regions of tensilestress, while it is hindered at the interface between the coatingand the substrate (Figs. 10 and 13). Local plastic deformation inthe shear bands will also change the local stress field and causeeventually crack initiation and crack growth. The observed shiftin the SN data to higher strain amplitudes is thus only a fractionof the compressive residual strain level in the coating. Further-

Fig. 13. Schematic model of the observed mechanism in the a-C:H coatedspecimens. The assumed residual stress distribution is shown on the top rightside. The magnitudes of residual strain and loading amplitudes are given in themanuscript.

6 J. Schaufler et al. / International Journal of Fatigue 37 (2012) 1–7

more, it is expected that the present surface defects will also re-duce the positive influence of the compressive residual strain onthe fatigue lifetime.

Similar approaches are discussed in literature based on Mor-rows analysis [28] on the effect of a mean stress on the fatigue life-time, where the stress amplitude is shifted by the amount ofresidual stress in the specimen [5]. In these models, the full mag-nitude of the residual stress is used for estimating the fatiguelife-time. For a coated specimen, it is however not clear, to whichmagnitude the residual stress in the coating will shift the fatiguelifetime of the specimen. As has been shown above, fully account-ing for the residual stress can lead to an overestimation of thefatigue lifetime.

The influence of the residual stress on the fatigue lifetime is alsonicely verified by the results of the pre-straining tests. By plasti-cally pre-straining the substrate, cracks were introduced in thecoating. Doing so, a drastically decreased fatigue life is observed,since crack initiation and crack growth can easily start at the pre-defined crack initiation sites [16] at the surface and is not hinderedby the coating. The compressive residual stress in the a-C:H coat-ing in combination with the good adhesion to the substrate, leadsthus to a strong reduction in surface roughening and to theenhanced fatigue life.

In Fig. 13 the relevant damage mechanisms in the a-C:H coatedspecimen schematically illustrated. The coating has distinct effectson the fatigue crack initiation as well as on the fatigue crack prop-agation. The typical fatigue cracks, initiating at extrusions can befully prevented. However subsurface fatigue crack initiation atshear bands is observed, indicating a tensile stress regime in thearea Cr layer/interface steel (see Fig. 13). Subsurface fatigue cracknucleation was also observed in the work of Wagner [3] on shotpeened Ti-alloys. Hereby, subsurface crack nucleation occurs dueto a tensile stress balance close to the outer compressive stressfield of the surface region. The crack closure in the region of theramp layer indicates a compressive stress level in the upper regionof the coating system, namely ramp layer and a-C:H. This shift inresidual stress can be correlated nicely with existing results onthe failure analysis of these coating systems. As shown in [29],the ramp layer in a Cr-based adhesion layer is a very sensitivestructural part in case of adhesion strength and stability againstloading. In accordance to the present results a shift of the residualstresses in the ramp layer can be assumed. This assumption isschematically included in the model in Fig. 13.

5. Conclusions

The influence of an a-C:H coating on the life time of an austen-itic stainless steel under cyclic loading up to Nf < 2,30,000 wasevaluated in this work. A strong increase in lifetime of more than300% for total strain amplitudes smaller than 4 � 10�3 is found.

In combination with the microstructural investigations the follow-ing conclusions can be given:

1. Shear band propagation which leads to extrusion and intrusionson the surface is hindered by the a-C:H coating, thus preventinga typical fatigue crack initiation at the surface. However, somesurface roughening of the coating due to fatigue loading isfound, indicating a shear band formation in the a-C:H coating.

2. The a-C:H coating can thus reduce the negative influence of sur-face defects (precipitations, pores) on the fatigue life.

3. For coated samples, subsurface cracks are found at shear bandsclose to the interface between the steel and the coating. Thesecracks do not penetrate into the coating and do not propagatealong the interface.

This behavior can be understood by considering the good inter-facial strength of the coating as well as the high compressive resid-ual stress level in the coating. Even at the applied tensile strainamplitudes, the coating is still in compression. However, the ob-served benefit in the total strain amplitude of the coated speci-mens is only a fraction of the residual strain level in the coating.This can be explained by the complex residual stress fields beneaththe coating. Here complementary tensile forces might cause crackinitiation and propagation beneath the surface. Moreover, it isshown, that the applied strain must not exceed the fracture strainof the coating. Normal cracks in the coating can then act as fatiguecrack nuclei and lead to a strong reduction in the fatigue lifetime.

Acknowledgement

The authors gratefully acknowledge the funding of the GermanResearch Council (DFG), which, within the framework of its ‘Excel-lence Initiative’ supports the cluster of Excellence ‘Engineering ofAdvanced Materials’ at the University of Erlangen-Nürnberg.

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